There is known a balancer shaft device which reduces rotational vibration of a crankshaft of an engine. An example of the balancer shaft device is disclosed in Patent Document 1. The balancer shaft device of Patent Document 1 includes a balancer shaft which serves to offset periodical rotational vibrations of a crankshaft of an engine, and the balancer shaft includes: a shaft portion which is disposed parallel to the crankshaft of the engine; and a balance weight (an eccentric weight portion) which is formed integrally with the shaft portion so as to be decentered from an axis of the shaft portion. The balancer shaft device further includes a rolling bearing which supports the shaft portion of the balancer shaft so as to be rotatable relative to an engine main body.
As the balance weight of the balancer shaft rotates, a centrifugal force is produced in a radial direction of the shaft portion. The centrifugal force may create a bending moment loaded on the shaft portion of the balancer shaft, which may reduce bending rigidity of the shaft portion. In order to increase the rigidity of the balancer shaft, there is known a balancer shaft device (see Patent Documents 2 to 4). The balancer shaft device includes a rib which is erected to extend, as viewed in a radial cross-section of the balancer shaft, radially outward from a portion of a shaft portion where a balance weight (an eccentric weight portion) is not disposed, i.e., from a portion of the shaft portion which does not contributed to offsetting of rotational vibration of a crankshaft (a non-eccentric weight portion).